Semiconductor memory, such as a random access memory (RAM), is an essential semiconductor device. A RAM device allows the user to execute both read and write operations on its memory cells. DRAM is a specific category of RAM containing an array of individual memory cells. DRAM devices are the most cost effective high speed memory used with computers and computer systems. Typically, each cell includes a capacitor for holding a charge and a transistor for accessing the charge held in the capacitor. The transistor is often referred to as the access transistor or the transfer device of the DRAM cell.
FIG. 1 illustrates a portion of a DRAM memory circuit containing two neighboring DRAM cells 100. Each cell 100 contains a storage capacitor 104 and an access field effect transistor (FET) 102. For each cell, one side of the storage capacitor 104 is connected to a reference voltage (illustrated as a ground potential for convenience purposes). The other side of the storage capacitor 104 is connected to the drain of the transfer device 102. The gate of the transfer device 102 is connected to a line known in the art as a word line 108. The source of the transfer device 102 is connected to a line known in the art as a bit line 106 (also known in the art as a digit line). With the memory cell 100 components connected in this manner, it is apparent that the word line 108 controls access to the storage capacitor 104 by allowing or preventing the signal (representing a logic “0” or a logic “1”) carried on the bit line 106 to be written to or read from the storage capacitor 104. Thus, each cell 100 contains one bit of data (i.e., a “0” or “1”).
As DRAM devices continue to be scaled down in size, it is difficult to provide capacitors in a small area with sufficient capacitance, typically greater than 30 femtoFarads (fF). In addition, it is difficult to provide an access transistor with good off-state leakage characteristics for refresh operations and good on-state characteristics to write into the cell. Several designs have been proposed to address these issues.
One such design is a silicon-on-insulator (SOI) based memory cell that eliminates the need for a capacitor. See K. Inoh et al, “FBC (Floating Body Cell) for Embedded DRAM on SOI,” 2003 Symp. on VLSI Tech. Digest, June 2003; P. Fazan et al., “Capacitor-less 1-T DRAM,” 2002 IEEE Int'l. SOI Conf, pp. 10-13, October 2002; H. Wann et al., “A Capacitorless DRAM Cell on SOI Substrate,” Tech. Digest, Int'l Electron Device Mtg., pp. 635-638, December 1993. The above references discuss one-transistor capacitor-less (1T/0C) DRAM cells and the operation of a DRAM circuit employing such cells. The above references are incorporated herein by reference.
Such capacitor-less cells, however, can suffer from poor performance characteristics related to retention time, access time, distribution characteristics, and reliability. In a 1T/0C DRAM cell, carriers are generated in the substrate bulk to write a “1,” and are pulled out from the substrate bulk to write a “0.” In a 1T/0C DRAM cell employing a planar SOI device, carrier generation can present problems. For example, when impact ionization is essential for operation of such a DRAM cell, device reliability can be poor and efficiency can be reduced at higher temperatures due to a decrease in ionization. Also, a planar device can result in limited operation speed, disturb, and write operations that consume a lot of power because the transistor must be in an on-state. Further, when the planar SOI devices are scaled to smaller sizes charge storage can be limited due to the reduced active area.
It would be advantageous to provide a storage device structure for use in a memory cell that would allow for reduced size while providing improved performance characteristics.